1. Field of the Invention
This invention relates to bipolar random access memories (often called "RAMs") and bipolar programmable read-only memories (often called "PROMs") and in particular to a structure wherein an epitaxial layer of one conductivity type is used with a substrate of opposite conductivity type together with oxide isolation thereby allowing Schottky diodes and transistors to be used in RAMs, and junction-programmable PROMs to be made in an oxide isolated structure and to a process for making these structures.
2. Prior Art
Various ways have been proposed to isolate electrically a plurality of pockets of semiconductor material in each of which one or more circuit elements can be formed. A number of these techniques are summarized in the introduction to U.S. Pat. No. 3,648,125 issued Mar. 7, 1972 on an invention of Peltzer. Two goals of these various isolation methods have been to reduce the size of the isolation regions as a proportion of the total area of silicon available for the formation of active devices and to decrease the size of the active devices. A major improvement in packing density over the packing densities achieved by the disclosures of these prior art patents was achieved by the method and structure disclosed by Peltzer wherein a thin silicon epitaxial layer was subdivided into electrically isolated pockets by a grid of oxidized regions of the epitaxial silicon material extending through the epitaxial material to an underlying laterally-extending isolation PN junction. Peltzer recognized that the use of diffused regions for isolation and for the formation of active semiconductor devices resulted in larger, less well-defined circuits than desired for many applications. Therefore, Peltzer replaced the diffused isolation regions with field oxide.
Among the different types of semiconductor devices which have been fabricated using the Peltzer oxide isolation technique are bipolar memories and programmable read-only memories. Programmable read-only memories are well-known in the semiconductor arts. One of the first patents disclosing a programmable semiconductor memory was Price U.S. Pat. No. 3,191,151 issued June 22, 1965 disclosing a programmable diode array. The Price array comprised a plurality of pairs of two diodes connected back-to-back. The array was programmed by sending a current pulse of the proper magnitude and direction through selected ones of the back-to-back diodes thereby destroying the junction of each of these back-biased diodes without affecting the connected forward-biased diode. The pulse converted the junction of each selected back-biased diode to a short circuit. Price also disclosed the use of a fuse which when destroyed converted the circuit from conducting to non-conducting. Additional patents disclosing programmable read-only memories include U.S. Pat. Nos. 3,733,690; 3,742,592; and 3,848,238. These patents disclose a programmable read-only memory also using avalanche breakdown to program selected devices and a method for achieving avalanche breakdown as in the Price U.S. Pat. No. 3,191,151.
However, heretofore it has not been feasible to combine the avalanche breakdown of a PN junction with the recessed oxide isolation structure of the prior art to yield a programmable read only memory programmable by avalanche breakdown of a PN junction because the voltages required to break down such a junction were close to the breakdown voltages of oxide isolated structures.